A ball joint can slide smoothly like a joint and change two joined members into arbitrary angles so as to transmit movement. Thus, such ball joints are used widely in the fields of auto parts, machine parts, controllers, and the like. Specifically, ball joints are applied to link mechanisms such as an arm, a tie rod, and a stabilizer in a suspension or a steering of an automobile or to movable parts of a transmission or the like.
In such ball joints, in recent years, for the purpose of further improvement in the wear resistance, the corrosion resistance, the sliding characteristics, and a friction behavior, an amorphous hard carbon film is formed on the surface of a ball stud spherical surface part serving as a substrate, as shown in FIG. 15.
This amorphous hard carbon film is generally referred to by various names such as diamond-like carbon (DLC), amorphous carbon, and diamond-form carbon (simply referred to as a “DLC film”, in some cases hereinafter). Such amorphous hard carbon films have a high hardness, a wear resistance, a solid-state lubrication property, and an excellent chemical stability as well as smoothness, low counterpart aggressiveness, and a self-lubricity and hence can preferably be used as sliding materials.
As for the above-mentioned formation of an amorphous hard carbon film on the surface of a ball stud spherical surface part, for example, Patent Document 1 disclose a technique that a DLC film having a nano indentation hardness of 6 to 39 GPa and a surface roughness (a root mean square roughness) of 60 nm or smaller is formed on a ball joint surface so that a ball joint provided with a DLC film having a low counterpart aggressiveness, a high corrosion resistance, and a high wear resistance is formed and hence occurrence of stick slip can sufficiently be suppressed and sliding characteristics such as torque characteristics can stably be obtained.